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The Amdo microcontinent which separates the Qiangtang terrane to the north and the Lhasa terrane to the south is a key terrane for reconstructing the tectonic evolution of Central Tibet. We report the new finding of retrograde eclogites within the Amdo microcontinent in this study. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile and quartz and underwent a four‐stage metamorphic evolution, including a peak eclogite facies stage (M1) at ~20–24 kbar and 580–620°C, followed by an HP granulite facies decompression stage (M2) at ~13–15 kbar and 750–780°C, a subsequent MP‐UHT granulite facies heating stage (M3) at 8–10 kbar and >840°C and a final amphibolite facies retrogression (M4) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element distribution patterns and trace element abundances similar to those of N‐MORB and arc‐related volcanics, with depleted whole‐rock εNd(t) values of 3.4 to 4.2, and are inferred to have formed in a back‐arc basin tectonic setting. Zircon and rutile U–Pb dating yields a protolith age of 226 ± 5 Ma, a peak eclogite facies metamorphic age of 190 ± 1 Ma, an HP granulite facies metamorphic age of 179 ± 1 Ma and an amphibolite facies retrograde age of 172 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back arc basin was subducted to depths of ~80 km. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back‐arc basin and the absence of arc magmatic rocks in the northern Amdo microcontinent. [ABSTRACT FROM AUTHOR]

The high‐pressure and low‐temperature metamorphic rocks constitute the important parts of the Longmu Co‐Shuanghu suture zone (LSS) in Tibet, providing a natural window for unveiling the tectonic evolution of the Palaeo‐Tethys Ocean. In this contribution, geochemical studies, including the whole‐rock geochemistry, Sr‐Nd isotopes and zircon U–Pb geochronology, were performed on the Pianshishan garnet amphibolites and retrograded eclogites, to reconstruct the genesis of the protoliths and to reveal tectonic evolution of the Longmu Co‐Shuanghu Palaeo‐Tethys Ocean (LSPTO). The garnet amphibolites show characteristics similar to the retrograded eclogites, representing different degrees of retrograde metamorphism of the Pianshishan eclogites. The protoliths of garnet amphibolites and retrograded eclogites are alkaline basalts, with geochemical characteristics similar to ocean island basalts (OIB) without Nb, Ta or Ti negative anomalies. Furthermore, geochemical data imply that the protoliths were formed by fractional crystallization of the magma derived from partial melting of garnet lherzolites without crustal contamination. The parental magma was probably generated in a relatively depleted but heterogeneous asthenosphere mantle source. Their protoliths erupted as a seamount within the LSPTO. Garnet amphibolites and retrograded eclogites yielded zircon U–Pb weighted mean ages of 240.2 ± 1.7 Ma and 239.1 ± 2.7 Ma, respectively. Combining with previous studies, we suggest that the LSPTO remained open in the Pianshishan area during the Middle Triassic, and the whole ocean might be gradually closed during the Early to Late Triassic. The small remnant ocean basin during the Middle Triassic can well explain a rapid evolution of the Pianshishan eclogite‐type rocks, that is, from generation to eclogite‐facies metamorphism and then to exhumation‐related retrograde metamorphism within ~19 Myr. [ABSTRACT FROM AUTHOR]

The Duguer area represents one of the few occurrences of high-grade metamorphic rocks in the ‘Central Uplift’ zone of the Qiangtang terrane, central Tibet. The metamorphic rocks consist mainly of orthogneiss, paragneiss, and schist. To better understand the formation of these rocks, seven samples of gneiss and schist from the Duguer area were selected forin situzircon U–Pb analysis and Ar–Ar dating of metamorphic minerals. The results suggest two distinct metamorphic stages, during the Late Triassic (229–227 Ma) and Late Jurassic (150–149 Ma). These stages correspond to the closure of the Palaeo-Tethys Ocean and northward subduction of the Bangong–Nujiang Neo-Tethys oceanic crust, respectively. We suggest that the Late Triassic metamorphic rocks of the Duguer area in the central South Qiangtang subterrane provide evidence of continental collision between the North and South Qiangtang subterranes, following the subduction of oceanic crust. It is likely that deep subduction of oceanic crust occurred along the Longmu Co–Shuanghu–Lancangjiang suture zone (LSLSZ), which would have hindered exhumation owing to the high density of oceanic crust. Subsequent break-off and delamination of the subducted oceanic slab at ~220 Ma may have resulted in exhumation of high-pressure and high-grade metamorphic rocks in the South Qiangtang subterrane. The Late Jurassic ages of metamorphism and deformation obtained in this study indicate the occurrence of an Andean-type orogenic event within the South Qiangtang subterrane. This hypothesis is further supported by an apparent age gap in magmatic activity (150–130 Ma) along the magmatic arc, and the absence of Late Jurassic sediments. [ABSTRACT FROM AUTHOR]

The Tibet Plateau was generated from a long-term, multiple and complex convergence of many micro-blocks and the final Indian-Eurasian collision, which records the evolution of the eastern Neo-Tethys Ocean. In order to clarify the evolution of the eastern Neo-Tethys, we reviewed the Bangong-Nujiang Suture Zone (BNSZ) according to the geological and geophysical evidence. We analyzed the BNSZ deep structure by geophysical data, and subduction polarity and the closure process of the Bangong-Nujiang Ocean by combining geophysical and geochronological data. Bouguer gravity anomaly is used to identify tectonic units on both sides of the BNSZ. We used multi-scale wavelet decomposition to separate the gravity anomaly at different depths for comparative analysis. Subsequently, the Moho discontinuity was estimated by gravity inversion. The inversion results show that the Moho depth varies from south to north beneath the Tibet Plateau, indicating a compression environment in a N S direction during the collision of micro-blocks. The collision caused uplift of the Tibet Plateau and an increase in crustal thickness. There is a clear Moho gradient zone along the BNSZ, and Moho depth changes more than 5 km between the South Qiangtang Terrane (SQT) and North Lhasa Terrane (NLT). The relatively shallow Moho surface of SQT indicates that the SQT experienced crustal thinning, which probably can be attributed to the different delamination of the lithosphere beneath the NLT and SQT. The nearly E–W-trending gravity gradient zone correlates well with the BNSZ. There are several gravity lows south of the BNSZ, that correspond well with outcrops of volcanic rocks, which developed from the Late Jurassic to Early Cretaceous, indicating that the Bangong-Nujiang Ocean may have subducted southward at that time. The BNSZ is a gravity gradient zone in the central Tibet Plateau, and the gravity field is very different on both sides of the BNSZ, which may be an important tectonic boundary. A combined analysis of gravity anomaly and ophiolite characteristics reveals different evolution of the eastern, central and western BNSZ. The evolution of the eastern Neo-Tethys consists of Carboniferous-Permian breakup, Triassic-Early Jurassic extension, Middle-Late Jurassic subduction and Cretaceous extinction. In particular, the closure of the Bangong-Nujiang Ocean represents the extinction of the eastern Neo-Tethys, which closed in the Yanshanian Period (Jurassic to Cretaceous), with scissor-style movement from east to west. The eastern BNSZ closed in the Middle Jurassic, while the central and western BNSZ closed in the late Early Cretaceous, and the collision-related deformation occurred mainly in the Cretaceous. • The nature of the Bangong-Nujiang Suture Zone (BNSZ), the subduction polarity and closing time of the Bangong-Nujiang Ocean are revealed from a geophysical perspective. • There are clear offsets on the Moho discontinuity of both sides of the BNSZ which obtained by gravity inversion. • A scissor-like closure from east to west of the Bangong-Nujiang Ocean during the Yanshanian Period is proposed. [ABSTRACT FROM AUTHOR]

The tectono-magmatic evolution of the Bangong-Nujiang Tethyan Ocean (BNO) plays a crucial role in understanding the Tibetan Plateau accretion before the Cenozoic India-Asia collision. The microcontinents and oceanic plateau recognized recently within the BNO provide important clues for constructing the subduction process between BNO and the Qiangtang terrane. In this contribution, we provide a comprehensive overview of the multidisciplinary information from magmatism, metamorphism, stratigraphy, geochronology, geochemistry, Hf isotopic data, ophiolite suites, and paleomagnetism to gain insights into the spatially and temporally related geodynamic processes of the BNO. A compilation of geochronological data indicates that the emplacement of the diverse igneous suites occurred mainly during three stages: stage 1 (190–150 Ma), stage 2 (130–110 Ma), and stage 3 (110–70 Ma). The igneous suites, including granitoids, mafic intrusions and volcanic rocks, formed two arcuate E–W trending belts that parallel the Bangong-Nujiang suture zone and are correlated to the double sided subduction of the BNO. We infer that the BNO initially opened at ~270Ma between the Lhasa terrane and Qiangtang terrane, although the relationship between the BNO and Longmu Tso-Shuanghu paleo-Tethyan ocean is still unclear. Age data related to metamorphism, arc magmatism, supra-subduction zoneophiolites, and sedimentary strata indicate that the northward subduction of the BNO started at ~190 Ma, followed by the subduction of microcontinents beneath the Qiangtang terrane at 195 Ma. In contrast, the subsequent southward subduction of the BNO started at ~170 Ma, resulting in the opening of the Shiquanhe-Namuco ocean. As indicated by the high Mg volcanic suite, intra-ocean subduction within the BNO also occurred during 170-145Ma, possibly related to the resistance from subduction of ocean plateau and slab retreat. Based on evidence from petrology, isotope geochronology, and isotopic geochemistry, we also infer that the magmatic lull during 145-130Ma is related to the slab subduction of ocean plateau beneath the southern Qiangtang subterrane. The closure of the BNO was most likely diachronous, which started as early as Late Jurassic and continued to the late Early Cretaceous (120-110Ma). • Initial opening of BNO at ~270 Ma and northward subduction at ~190 Ma. • Southward subduction started at ~170 Ma, leading to the opening of SNO. • Ocean plateau mechanism for intra-ocean subduction of BNO and magmatic lull during 145–130 Ma. • Diachronous closure of the BNO initiated at 150 Ma and the final closure during 110–100 Ma. [ABSTRACT FROM AUTHOR]

• Five stages of magmastism occurred along both sides of the BNS. • The Early Jurassic and the Late Cretaceous granitoids are produced by roll back and extension-collapse of the Tongka micro-block, separately. • The collision between Lhasa and Qiangtang terranes may continue to the latest Cretaceous (<74 Ma). • Ophiolites north of Amdo, Baqing and Dengqen were possiable lithospheric remanants of the LSO. The Tongka micro-block, located in the eastern section of the Bangong-Nujiang suture, and characterized by Mesozoic granitoids and high grade metamorphic rocks, holds the key to the evolution of the Bangong-Nujiang ocean (BNO). Here we present a systematic petrologic, geochemical, zircon U-Pb-Hf, and whole rock Sr-Nd-Pb isotopic investigation on six new granitoids in Tongka. The results suggest these granitoids yielded two stages of crystallization (171–177 Ma and 74 Ma) and two stages of metamorphism (201–215 Ma and 170–175 Ma). Combined with the existing work within BNS, the magmatism along the BNS can be divided into five stages: stage 1 (240–250 Ma), stage 2 (200–230 Ma), stage 3 (150–190 Ma), stage 4 (100–135 Ma) and stage 5 (73–100 Ma). Both the Early Jurassic and Late Cretaceous Tongka granitoids in this study are high-K calc-alkaline series, with enrichments in large ion lithophile elements (e.g., Cs, Rb, and K) and depletions of large ion lithophile elements (e.g., Nb, Ta, and Ti). The variable ε Hf (t) values (−17.3 to −7.6), ε Nd (t) values (−12.08 to −11.15) and ancient T DM2 (1.48–2.00 Ga) for the Early Jurassic granitoids and weakly negative ε Hf (t) values (−6.3 to −1.5), ε Nd (t) values (−8.75 to −8.60) and old T DM2 ages (1.07–1.33 Ga) for the Late Cretaceous granitoids, suggest that they are both mainly derived from the melting of ancient crust with minor mantle material input. However, the formation of the Early Jurassic and Late Cretaceous granitoids in Tongka is related to different tectonic settings: roll back after the Tongka micro-block subducted beneath the southern Qiangtang subterrane and extension-collapse after the Tongka micro-block collided with the northern Lhasa subterrane. The initial subduction of the BNO may start from 190 Ma, and the collision between the Lhasa and Qiangtang terranes may continute to the latest Cretaceous (<74 Ma). Ophiolites north of Amdo, Baqing and Dengqen were possiable lithospheric remanants of the Longmu Tso-Shuanghu ocean (LSO) rather than the BNO. [ABSTRACT FROM AUTHOR]